A cathode ray tube (CRT) is a display technology device which is earliest to realize industrial production and most widely applied, has the advantages of mature technology, high reliability, long service life and the like, and is taken as a main display device of televisions, computer displays, oscilloscopes and other electronic devices. At present, the social possessing capacity of the existing televisions in China exceeds 0.4 billion, wherein most of the televisions are CRT televisions, and the possessing capacity of the computer CRT displays has also exceeded 40 millions. With the rapid development of an electronic display device technology, the liquid crystal displays, due to the peculiar superiority thereof, seriously assault the market share of the CRT displays, so that the amount of the abandoned CRT displays is increased year by year. China has developed into a big country producing and consuming electronic products, and a large number of electronic products have entered a peak period of elimination and obsolescence. The ‘old-for-new’ policy for household electrical products had been promoted and implemented in China since Jun. 1, 2009, and a total of about 15 million waste electronic appliance products had been recovered up to the end of May, 2010, wherein the CRT televisions accounted for about 82%. About 12 million displays were recovered in total in 28 pilot provinces and cities in China, and almost all the displays were the CRT displays.
The lead-containing CRT glass in the waste CRT belongs to the hazardous waste and has become the focus of attention of treatment of the electronic wastes. At present, landfill is still taken as a main way for treatment and disposal of the waste CRT glass in part of countries or regions. However, when the CRT glass is present in a landfill field over time, heavy metal lead in the waste CRT glass will be dissolved out and enter groundwater and further bring serious harm to ecological environment and human health. In one report of US environmental protection agency, it is written that 98.7% of metallic lead in municipal solid wastes is from the electronic wastes, wherein 29.8% is from the waste CRT glass. Stephen, et al., adopted a solid waste TCLP (Toxicity Characteristic Leaching Procedure) recommended by US environmental protection agency to perform a leaching toxicity test on the lead in the waste CRT funnel glass, and found according to the results that the leaching concentration average value of the lead in the CRT funnel glass was about 75.3 mg/L, which was far beyond the identification standard for hazardous wastes. Thus, how to dispose and utilize a huge number of waste CRT displays has become a major challenge in the field of environmental protection in China and even all over the world.
At the same time, the CRT glass is an unignorable resource solid waste. It is estimated that the weight of the CRT glass which is currently used in the whole country is about 6 million tons, wherein the total lead content accounts for about 8.3%, reaching 0.5 million ton. At present, in the industry of smelting, raw materials for lead smelting mainly comprise lead sulfide ores, and the grade of the mined ores is lower than 3% generally; furthermore, the ores need to be subjected to ore dressing to obtain a lead concentrate, and then smelting is performed; and the lead grade in the colored CRT funnel glass is about 20 wt. %.
By comprehensively considering the factors in the aspects of resources, environment, technologies, economy and the like, resource recycling of the waste lead-containing CRT glass is a relatively reasonable disposal way. At present, the researches of lead extraction treatment methods for the waste CRT glass around the world are mainly focused on the following four types: (1) soaking and washing with acid and alkali solutions; (2) extracting the metallic lead by thermal treatment; (3) performing mechanical extraction; and (4) performing thermal treatment, washing, soaking and other steps for mixing. Wherein, by adopting the method of soaking and washing with acid and alkali solutions, the lead extraction efficiency is relatively low. According to a paper titled “Innovated Application of Mechanical Activation to Separate Lead from Scrap Cathode Ray Tube Funnel Glass” published in 2012 in Environmental Science & Technology (vol. 46, pp. 4109-4114), a strong acid soaking and washing method was adopted, but the achieved lead ion recovery rate was only 1.2% (by dissolving 0.5 g of CRT glass powder in 75 ml of 3 mol/l nitric acid solution and heating at 95° C. for 2 hours).
The methods for extracting the metallic lead by thermal treatment have been greatly researched, including the following three methods: thermal vacuum reduction with carbon, thermal reduction with SiC and TiN and high-temperature self-propagating evaporation treatment. According to the method of thermal vacuum reduction with carbon, carbon powder needs to be used as a reducing agent, and the metallic lead is extracted from a glass body by performing the thermal treatment process at the temperature of 1000° C. and low vacuum of 10-100 Pa for at least four hours. Due to the relatively high requirements for thermal treatment temperature and vacuum degree, such method is very difficult to realize large-scale industrial application. Relatively speaking, thermal reduction of the metallic lead with SiC and TiN from the CRT glass powder requires lower temperature (about 850° C.), but the extraction rate is also lower (<40%), and the reducing agents (SiC and TiN) adopted in such reduction method are expensive. By adopting the self-propagating high-temperature evaporation method for recovering the metallic lead, metallic Mg can spontaneously react with Fe2O3 to provide heat at the ultra-high temperature, thereby performing the self-propagating process. When the saturated vapor pressure of the metallic lead achieves the level for evaporation of the lead, the metallic lead can be evaporated from the glass body. Generally speaking, by adopting the method, the required temperature is about 2000° C., and the cost for extracting the metallic lead with the metallic Mg is also higher.
The mechanical extraction method taking Na2EDTA as an extracting agent requires longer extraction time (20 h) and higher consumption of the extracting agent Na2EDTA. Another reported method is to perform sulfurization on the mechanically activated lead-containing CRT glass powder with elemental sulfur, but the sulfurization rate of the metallic lead has not been reported, and the final product of the method is PbS but not metallic lead.
Till now, the technical methods for recovering the lead from the lead-containing CRT glass waste require relatively high energy consumption to damage the 3-D glass structure embedded with the lead and further extract the metallic lead. This is because that lead atoms are included in the structure of PbO3 polyhedrons and the PbO3 polyhedrons are further firmly sealed by SiO4 tetrahedrons to form a dense reticular glass structure. The lead content in the CRT funnel glass is higher than that in the lead ores, but the main difficulty is a lack of effective and feasible technology for extracting the lead from the glass with the reticular silicate structure. According to the current literature, it is found that the technologies for extracting the lead often require higher energy, expensive chemicals or complex extraction processes. Generally speaking, the temperature required for extracting the metallic lead from the SiO2 reticular glass structure is higher than 1000° C.